Calibration Reference Data System

COS Reference File Tables

Reference files used by the COS calibration pipeline
(CALCOS)) are updated on a regular basis. Current
reference files files can be retrieved from the HST archive, or downloaded
directly from the
lref directory. If you choose to access the files via this ftp server, keep in mind that there is
a known bug in trying to access files via ftp server using Safari. The recommended browser is Firefox.

Flat Field Files (FLATFILE)

Accounts for pixel-to-pixel variations.

FLATFILE provides a flat field image which is used by the pipeline to remove the
pixel-to-pixel variations in the detector. The FUV FLATFILE consists of a primary
header and two IMAGE extensions, one for each segment. The NUV FLATFILE consists
of a primary header and a 1024 x 1024 IMAGE extension.

Currently (08/2013), the FUV flat field corrects the grid wires for the G130M and G160M.
The grid wires are flagged but not corrected in G140L. With the OPUS 2013.2 installation,
expected in Sept 2013, there will be a single flat field applied to all FUV observations
(including G140L) which corrects the grid wires as well as edge features on segment B
caused by inaccuracies in the geometric distortion correction.

The NUV flat field is a combination of internal and external deuterium flat field lamp
exposures from thermal-vacuum testing which illuminate the portion of the detector that
will receive all of the incoming external light on orbit. The data cover the following
pixel region of the detector: x (dispersion): 0 to 1023, and y (cross-dispersion): 495 to
964. The rest of the detector, where flat field data are not available, has a value of 1.0.
The bottom four and top three rows of the detector do not fit well with the rest of the
detector and they are flagged in the data quality table.

Geometric Distortion Correction Images (GEOFILE)

Corrects for the intrinsic nonlinearity (INL) of the FUV detector.

This file is only used for FUV data. The GEOFILE is used by the
GEOCORR calibration module to perform the geometric correction. From
the nature and construction of the XDL detectors, the physical size of the
pixels vary across the detector. The geometric distortion maps are used to
correct for this variation and to transform the data into a constant physical
pixel size early in the data reduction calibration process. After the thermal
correction has been applied and the detector digital span and position are
adjusted to their reference values, as defined in the reference table, the
geometric correction can be applied. This implies that all the files used to
determine the geometric correction were initially thermally-corrected.

Each geometric correction reference file contains four IMAGE
extensions. There are two for each segment, and for each segment, there is
one for each axis. At a given (X,Y) location in the uncorrected COS data,
the value at that location (corrected for binning and offset) in the geometric
correction image gives the distortion to be subtracted from the X or Y
coordinates.

Delta Geometric Distortion Correction Image (DGEOFILE)

Improves the geometric correction for the FUV detector.

The delta geometric distortion reference file is used to improve the geometric
correction for the FUV detector. It is defined and applied in the same way as
the Geometric Correction, and is only applied to data that have been geometrically
corrected. At a given (X,Y) location in the geometrically corrected COS data, the
value at that location (corrected for binning and offset) in the delta geometric
correction image gives the distortion to be subtracted from the X or Y coordinate.

X Walk Correction File (XWLKFILE)

Gives the correction for the pixel coordinates XCORR and YCORR of FUV TIME-TAG data.

The X walk correction file gives the correction for the pixel coordinates XCORR and YCORR of FUV
TIME-TAG data, depending on the XCORR value and pulse height amplitude (PHA). This correction, along
with the Y walk correction, supersedes the old WALK correction and became active after calcos
version 3.2.1 in June 2017. The reference file is an image with each pixel (i, j) containing the walk
correction for an event with XCORR value i and PHA value j, where i and j correspond to the indices of the
pixel in the x (most rapidly varying) direction and y (least rapidly varying) direction. Bilinear interpolation
is used to calculate the correction for non-integer values of XCORR and PHA, although the latter coordinate is
currently always an integer.

Y Walk Correction File (YWLKFILE)

Gives the correction for the pixel coordinates YCORR of FUV TIME-TAG data.

The Y walk correction file gives the correction for the pixel coordinates YCORR of FUV
TIME-TAG data, depending on the XCORR value and pulse height amplitude (PHA).
This correction, along with the X walk correction, supersedes the old WALK correction and became active
after calcos version 3.2.1 in June 2017. The reference file is an image with each pixel (i, j) containing
the walk correction for an event with XCORR value i and PHA value j, where i and j correspond to the indices
of the pixel in the x (most rapidly varying) direction and y (least rapidly varying) direction. Bilinear
interpolation is used to calculate the correction for non-integer values of XCORR and PHA, although the latter
coordinate is currently always an integer.

Bad Time Interval Table (BADTTAB)

Lists the start and end times of known bad time intervals.

The BADTTAB reference file lists the start and end times of known bad
time intervals. It is used by the BADTCORR calibration module to flag
events in TIME-TAG events lists which occur during a bad time interval. In
later processing the flagged events will be removed from the final
calibrated data, and the exposure time header keyword, EXPTIME,
updated. The bad time interval table consists of segment, start, and end
columns. The segment column can be populated with
either FUVA, FUVB or ANY; the start and end columns are in Modified
Julian Date.

Baseline Reference Frame Table (BRFTAB)

Used when applying the thermal distortion correction.

The BRFTAB reference file is only applicable to FUV data and is used
during pipeline processing in the TEMPCORR module to apply the
thermal distortion correction. The FUV detector does not have physical
pixels like a CCD. Instead, the x and y positions of detected photon events
are obtained from analog electronics, which are susceptible to thermal
changes. Electronic stim pulses are normally commanded during
integration and are used as physical position reference points. To return the
FUV data to a known physical space, the BRFTAB defines the stim
positions.

The BRFTAB file consists of a primary header extension and a binary
table extension. The table lists the stim locations, stim search regions, and
the active detector areas.

Burst Parameters Table (BRSTTAB)

Provides the parameters needed for deciding when
the count rate in some time interval in FUV TIME-TAG data is too high
to be regarded as normal.

The BRSTTAB file provides the parameters needed to identify bursts. It
consists of a primary header extension and a binary table extension with the
columns listed in Table 3.5 of the COS Data Handbook.

1-D Extraction Parameters Table (XTRACTAB)

Gives the location of the spectrum to be extracted from a 2-D image.

This table provides the information needed to extract
the spectrum from a geometrically corrected image of the detector for each
optical element and central wavelength.

Bad Pixel Table (BPIXTAB)

Gives the locations of rectangular regions that cover portions of the detector that
are known to be less than optimal.

The bad pixel table identifies rectangular regions on the
detectors that are known to be less than optimal. The feature type describes
the type of detector blemish enclosed within the bounding box and q is the
quality value assigned to all events detected within the box. The regions
were identified by visual inspection of the combined flat field data for each
detector (and segment).

Deadtime Reference Table (DEADTAB)

Used to obtain the true number of events received compared to the number of events counted by the
detector electronics.

The DEADTAB reference file is used in the DEADCORR module, to
obtain the true number of events received compared to the number of
events counted by the detector electronics.

There is one DEADTAB reference file for the NUV and FUV detectors.
They consist of a primary header extension and a binary table extension
which contains the livetime values for a given observed count rate and
segment. The livetime is defined as:

livetime = observed rate / true rate

and can be used to calculate the true count rate.

Dispersion Coefficients Table (DISPTAB)

Gives a set of polynomial coefficients for computing wavelength from pixel number.

This table provides the dispersion relations for each
segment, aperture, optical element and central wavelength.

For Px = the Doppler corrected pixel value in the dispersion direction,
the associated wavelength for a specific segment, optical element, aperture,
and central wavelength is given by:

The LAMPTAB file consists of a primary header extension and a binary
table extension which contains an extracted 1-D spectrum from the internal
PtNe calibration lamp through the WCA aperture, for each grating and
central wavelength setting. It is used in the calcos pipeline to determine the
pixel offset of the observed data.

Pulse Height Parameters Table (PHATAB)

Lists the lower and upper thresholds for valid individual pulse heights in TIME-TAG mode
as well as the minimum and maximum values for the location of the mean value of the pulse height
distribution used in ACCUM mode.

The PHATAB reference file is only valid for FUV data, and is applied
during the PHACORR step of calcos to filter non-photon events. The table lists the lower and upper
thresholds for valid individual pulse heights in TIME-TAG mode. In
TIME-TAG mode, each detector event has an associated pulse-height of 5
bits with values ranging from 0 to 31, The table also gives the minimum
and maximum values for the location of the mean value of the pulse height
distribution used in ACCUM mode. In ACCUM mode, a pulse height
distribution histogram is generated for the whole exposure and downloaded
as part of the science data file. The histogram includes all the digitized
events for each segment independently of the currently defined subarrays.
Note in ACCUM mode the pulse height is a 7 bit number with values
ranging from 0 to 127.

Photometric Sensitivity Reference Table (FLUXTAB)

Provides the information needed to convert from corrected detector counts to flux units.

This table provides the information needed to convert
from corrected detector counts to flux units of erg s-1cm-2A-1 for each
segment, optical element, aperture and central wavelength.

For each segment, optical element, central wavelength
setting, and aperture, these files contain arrays of wavelengths and
sensitivities which can be interpolated onto the observed wavelength grid.
The net counts can then be divided by the sensitivity curves to produce flux
calibrated spectra.

Spectroscopic WCS Parameters Table (SPWCSTAB)

The spectroscopic WCS parameters table gives values to be
used for populating the world coordinate keywords.

Files are selected on DETECTOR and OBSTYPE and only for SPECTROSCOPIC data

Time Dependent Sensitivity Table (TDSTAB)

Gives information about the relative sensitivity as a function of time and wavelength.

There are two such files, one for the FUV and one for the NUV and are
only used for spectroscopic data. The files contain the information
necessary to determine the relative sensitivity curve at any given time by
interpolating between relative sensitivity curves given at fiducial times
which bracket the observation, or else extrapolate the results from the last
curve if the observation date is more recent than the last fiducial date.
Interpolation data are provided for each segment, optical element, and
aperture.

Wavecal Parameters Table (WCPTAB)

Gives information about the relative sensitivity as a function of time and wavelength.

The WCPTAB file contains information relevant for the wavecal pipeline
processing. A fixed RESWIDTH (the number of pixels per resolution element in the dispersion
direction) value of
6.0 pixels (per resolution element) is used for the FUV detector and a fixed
RESWIDTH value of 3.0 pixels (per resolution element) is used for the
NUV detector. The FUV STEPSIZE (one step of the OSM is this many pixels) is measured by calculating the
displacement in pixels from a PtNe spectrum obtained at a position of
FPOFFSET=0 to the position FPOFFSET=-2 for segment A from the
WCA (and dividing by 2). The NUV STEPSIZE is measured by
calculating the displacement in pixels from a PtNe spectrum obtained at a
position of FPOFFSET=0 to the position FPOFFSET=-2 for stripe B of
the WCA (and dividing by 2). The XC_RANGE (the maximum lag (amplitude) for cross correlation) was estimated
as 110% of
the STEPSIZE for both FUV and NUV.

High Voltage Reference Table (HVTAB)

Gives the times when the FUV high voltage was changed, and the value (raw counts) that was used in the
command to set the high voltage.

Calcos will use the values in this table to populate extension header keywords HVLEVELA and HVLEVELB.

This file contains two extensions. Each extension is a binary table giving the date/time and commanded
high voltage value for one FUV segment.
Keyword EXTNAME is used for selecting the appropriate extension.

The parameters are used for computing the coefficients of polynomials in PHA (one polynomial for XCORR,
another for YCORR) that give the corrections to be subtracted from XCORR and YCORR. The coefficients of PHA
are themselves computed by evaluating polynomial functions of XCORR and YCORR. The changes to be subtracted
from XCORR and YCORR are:

Where N is the value in the N_PHA_COEFF column, x_c0 is the value in the x0 column, y_c0 is the value in the
y0 column, and x_ci and y_ci for i > 0 are the values in the XCOEFF and YCOEFF columns, respectively.

Gain Sag Reference Table (GSAGTAB)

Gives the locations of rectangular regions for portions of the FUV detector that have very
low pulse height amplitude (PHA).

For each such region, a data quality
flag value is given, although the value is the same in every row.

This file contains multiple extensions. Each extension is a
binary table giving regions to be flagged as low gain regions. The
appropriate extension to use is selected by SEGMENT and FUV high voltage
(given by keyword HVLEVELA or HVLEVELB in the first extension header of
the raw file); see also the section on extension header keywords below.
Each row of the table gives the location and data quality value for one
rectangular region. The format of a table is similar to the bad pixel
reference table (_bpix.fits), except that the gain sag table includes a
DATE column and does not include either a SEGMENT or TYPE column. The
DATE column is used to select rows. A row will be used to flag a gain
sagged region if the value in the DATE column is less than or equal to
the exposure start time.